Iron–titanium oxide-engineered biochar mitigates antimony and nickel bioavailability in paddy soil: implications for soil geochemistry, microbial dynamics, and rice grain safety

Co-contamination of paddy soils with antimony (Sb) and nickel (Ni) poses significant risks to soil quality, crop productivity, and food safety. This study evaluated the effectiveness of iron–titanium oxide-engineered biochar (Fe-Ti-BC) in mitigating Sb and Ni mobility in contaminated paddy soil and examined associated changes in soil carbon dynamics and microbial communities. Compared with the control, Fe-Ti-BC application reduced CaCl₂-extractable Sb and DTPA-extractable Ni by 20.31–34.31% and 39.87–79.67%, respectively. Amendment with Fe-Ti-BC and unmodified biochar enhanced the carbon pool management index (CPMI) and labile organic carbon fractions, indicating improved carbon sequestration potential and nutrient cycling capacity. Rice biomass significantly increased following biochar treatments. Iron and titanium concentrations in root Fe plaque were approximately 20-fold and twofold higher, respectively, in Fe-Ti-BC-amended soils than in the control, suggesting enhanced plaque-mediated immobilization of trace elements. Consequently, Sb and Ni concentrations in rice grains decreased by 20.02–78.18% and 60.17–69.79%, respectively. High-throughput sequencing revealed that Fe-Ti-BC reshaped soil bacterial community composition and metabolic activity, promoting keystone taxa including Actinobacteria, Proteobacteria, and Firmicutes. Partial least squares path modeling (PLS-PM) identified CaCl₂-extractable Sb, DTPA-extractable Ni, and Fe-Ti plaque formation as key determinants governing trace element accumulation in rice grains. Overall, Fe-Ti-BC effectively stabilized Sb and Ni through coupled geochemical and biological mechanisms, thereby reducing metal transfer to edible tissues while enhancing soil carbon functionality and productivity. These findings highlight the potential of engineered biochar as a sustainable remediation strategy for multi-metal contaminated paddy systems with direct implications for environmental health and food security.